The present invention relates to the magnetic resonance arts. It finds particular application in conjunction with whole body magnetic resonance imaging systems and will be described with particular reference thereto. However, it is to be appreciated that the invention may also find application in conjunction with small bore imaging systems, magnetic resonance spectroscopy systems, and the like.
Heretofore, medical magnetic resonance imaging systems have utilized analog transmitters and receivers. The transmitters commonly generated a radio frequency signal at the Larmor frequency of dipoles of interest in the imaging region. The radio frequency signal was shaped into pulses with selected frequency, phase, and amplitude characteristics and sent by way of a power amplifier to radio frequency transmitter coils. The pulse of frequency, phase, and amplitude modulation were effected with analog components.
Frequency modulation was accomplished with a frequency synthesizer which generated an analog signal of selectable frequency. Although some frequency synthesizers had digital sections, the resultant frequency signal was analog. Pulse amplitude modulation was commonly conducted with double balanced mixers and analog gates or switches.
Similarly phase modulation was achieved by phase shifting an intermediate radio frequency through the use of a phase splitter and combining it with an appropriate frequency to achieve the required Larmor frequency again through the use of a double balanced mixer. The use of these devices presented problems that double balanced mixers are inherently non-linear, e.g. square law devices, phase splitters had a wide error margin and analog gates tended to have leakage introducing unwanted characteristics in the transmitted radio frequency pulse. In order to operate the double balanced mixer in the most linear portion of its operating curve it was necessary to add external components to bias the input appropriately. Since the bias point could vary from unit to unit it was necessary to incorporate adjustable components. Similar methods were required to calibrate phase splitting devices and reduce leakage in analog gates. These adjustments not only increased the cost and complexity of the hardware and initial calibration, but also provided numerous unauthorized adjustment points.
The analog magnetic resonance signals emanating from the subject were received and demodulated by an analog based receiver. Analog phase sensitive detectors produced sine and cosine related channels of analog signals. The sine and cosine analog signals were separately digitized and digitally processed to perform fast Fourier transform and other digital processing operations to create an image representation.
Analog receivers were, again, non-linear and suffered the above described problems of non-linear components. Signals which bled through the analog components or devices became artifacts in the resultant image. The variations from device to device were more critical in the receiver. Failure to maintain channel to channel balance, amplitude, and phase consistency in the sine and cosine or real and imaginary channels also resulted in image artifacts. The analog receivers were further subject to DC level errors. Typically, twenty to thirty seconds were required before each scan in order to determine the actual, current DC level so that appropriate compensation could be made.
The present invention contemplates a new and improved magnetic resonance system which overcomes the above referenced problems and others.